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Bacterial Rheotaxis Bacterial rheotaxis Marcosa,b, Henry C. Fuc,d, Thomas R. Powersd, and Roman Stockere,1 aSchool of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore; bDepartment of Mechanical Engineering, and eDepartment of Civil and Environmental Engineering, Ralph M. Parsons Laboratory, Massachusetts Institute of Technology, Cambridge, MA 02139; cDepartment of Mechanical Engineering, University of Nevada, Reno, NV 89557; and dSchool of Engineering and Department of Physics, Brown University, Providence, RI 02912 Edited by T. C. Lubensky, University of Pennsylvania, Philadelphia, PA, and approved February 17, 2012 (received for review December 21, 2011) The motility of organisms is often directed in response to environ- passive hydrodynamic effect, whereby the combination of a mental stimuli. Rheotaxis is the directed movement resulting from gravitational torque and a shear-induced torque orients the fluid velocity gradients, long studied in fish, aquatic invertebrates, swimming direction preferentially upstream (14–16). Responses and spermatozoa. Using carefully controlled microfluidic flows, we to shear are also observed in copepods and dinoflagellates, which show that rheotaxis also occurs in bacteria. Excellent quantitative rely on shear detection to attack prey or escape predators (5, 17– agreement between experiments with Bacillus subtilis and a math- 19), orient in flow (20), and retain a preferential depth (21). ematical model reveals that bacterial rheotaxis is a purely physical Evidence of shear-driven motility in prokaryotes is limited to the phenomenon, in contrast to fish rheotaxis but in the same way as upstream motion of mycoplasma (22), E. coli (23), and Xylella sperm rheotaxis. This previously unrecognized bacterial taxis results fastidiosa (24), all of which require the presence of a solid sur- from a subtle interplay between velocity gradients and the helical face. In contrast, little is known about the effect of shear on shape of flagella, which together generate a torque that alters a bac- bacteria freely swimming in the bulk fluid. Using Bacillus subtilis terium’s swimming direction. Because this torque is independent of as a model organism, we here report that bacteria exhibit the presence of a nearby surface, bacterial rheotaxis is not limited to rheotaxis that is not conditional to the presence of a nearby the immediate neighborhood of liquid–solid interfaces, but also surface and we demonstrate that this phenomenon results from fl takes place in the bulk uid. We predict that rheotaxis occurs in the coupling of motility, shear, and cell morphology. a wide range of bacterial habitats, from the natural environment to the human body, and can interfere with chemotaxis, suggesting Results and Discussion fi fi that the tness bene t conferred by bacterial motility may be To expose bacteria to controlled shear flows, we injected a sus- sharply reduced in some hydrodynamic conditions. pension of the smooth-swimming B. subtilis OI4139 (25) (Materials and Methods)inanH =90-μm deep microfluidic channel. This low Reynolds number | directional motion | chirality strain has a 1 × 3-μm sausage-shaped body, multiple left-handed − helical flagella, and swims at U =40–65 μm·s 1. Bacteria were he effectiveness and benefit of motility are largely de- imaged a distance H/4 above the bottom of the microchannel, − Ttermined by the dependence of movement behavior on en- where the shear rate S was varied between 0 and 36 s 1. Cell- vironmental stimuli. For example, chemical stimuli may affect tracking software yielded the bacteria’s drift velocity, V, perpen- the spreading of tumor cells (1) and allow bacteria to increase dicular to the flow gradient plane (i.e., along z;Fig.1C and uptake by swimming toward larger nutrient concentrations (2, 3), Materials and Methods). whereas hydrodynamic stimuli can stifle phytoplankton migra- Experiments revealed a large and highly reproducible drift tion (4), allow protists to evade predators (5), and change velocity along –z (Fig. 2A), whose magnitude increased with in- sperm–egg encounter rates for external fertilizers (6). Micro- creasing shear rate. The drift velocity amounted to a considerable organisms exhibit a broad range of directed movement respon- fraction of the swimming speed, with V/U =6.5%± 2.4% (mean ± − − ses, called “taxes”. Whereas some of these responses, such as SD) at a shear rate of S =10s 1 and 22% ± 4% at S =36s 1. chemotaxis (7) and thermotaxis (8), are active and require the Furthermore, bacteria imaged one-quarter depth below the top ability to sense and respond to the stimulus, others, such as of the channel, where the shear rate had the same magnitude but magnetotaxis (9) and gyrotaxis (4), are passive and do not imply opposite sign, exhibited the same drift velocity, only in the op- sensing, instead resulting purely from external forces. posite direction (Fig. 2B). Taken together, these results indicate Chemotaxis is the best studied among these directional that the observed drift was caused by shear. motions: Bacteria measure chemical concentrations and migrate This drift is surprising because many objects, such as spheres along gradients (Fig. 1A). For instance, chemotaxis guides and ellipsoids, do not drift across streamlines at the low Reynolds Escherichia coli to epithelial cells in the human gastrointestinal numbers, Re = UFL/v < 0.02, characteristic of our experiments tract, favoring infection (10); Rhizobium bacteria to legume root (26) (L is the object’s length, UF the mean flow velocity, and v the hairs in soil, favoring nitrogen fixation (2); and marine bacteria kinematic viscosity of the fluid). However, this principle does not to organic matter, favoring remineralization (3). Equally as apply to chiral objects, like helices, which can drift in the di- pervasive as chemical gradients in microbial habitats are gra- rection perpendicular to the flow gradient plane (27, 28). This fl “ ” dients in ambient uid velocity or shear (Fig. 1B). Although effect was recently demonstrated for nonmotile spirochetes (29) fl — nearly every uid environment experiences velocity gradients and originates from the hydrodynamic stresses on a helix in flow, from laminar shear in bodily conduits and soil to turbulent shear which produce a net lift force on the helix that is normal to the in streams and oceans—the effect of velocity gradients on bac- terial motility has received negligible attention compared with fi chemical gradients, partly due to the dif culty of studying mo- Author contributions: M., H.C.F., T.R.P., and R.S. designed research; M. and H.C.F. per- tility under controlled flow conditions. formed research; M. and H.C.F. analyzed data; and M., H.C.F., T.R.P., and R.S. wrote Rheotaxis refers to changes in organism movement patterns the paper. due to shear. Rheotaxis is common in fish (11, 12), which actively The authors declare no conflict of interest. sense shear and respond by either turning into the flow or flee- This article is a PNAS Direct Submission. ing high-flow regions. Certain insects use rheotaxis to escape 1To whom correspondence should be addressed. E-mail: [email protected]. droughts by swimming upstream in desert rivers (13). At smaller This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. scales, spermatozoa exhibit rheotaxis, likely resulting from a 1073/pnas.1120955109/-/DCSupplemental. 4780–4785 | PNAS | March 27, 2012 | vol. 109 | no. 13 www.pnas.org/cgi/doi/10.1073/pnas.1120955109 Downloaded by guest on October 6, 2021 Fig. 1. Bacterial chemotaxis and rheotaxis. (A) Many bacteria can climb chemical gradients by chemotaxis, for example to seek high nutrient concentrations. (B) Ambient velocity gradients can also affect motility, potentially hampering chemotaxis by orienting bacteria away from nutrient sources. The x–y plane is the flow gradient plane, defined by the direction of the flow (x) and the direction of the flow velocity gradient (y). (C) The mechanism responsible for bacterial rheotaxis, shown for a cell with a left-handed flagellum. The chirality of the flagellum causes a lift force along +z. This force is opposed by the drag on the cell body, producing a torque on the cell. This torque reorients the bacterium, which therefore has a component V of its swimming velocity U directed along –z. V is the rheotactic velocity. flow gradient plane (Fig. 1C). This force—balanced by drag— drag forces and torques on a sphere of radius a moving with causes the helix to drift across streamlines. linear velocity v and angular velocity ω are given by 6πηav and The helical shape of bacterial flagella suggests that the same 8πηa3ω, respectively (32), where η is the dynamic viscosity of the phenomenon might be responsible for the observed drift. How- fluid. We use the fact that the translational drag in shear flow is ever, a model of the flagellar bundle of B. subtilis predicts a drift the same as in uniform flow and that the effect of shear flow on velocity that has the opposite direction (+z) to that observed (–z; rotational drag is to change the rotational rate only by an amount − Fig. 2A) and a magnitude of only 0.16 μm·s 1 (V/U = 0.29%) at equal to half the vorticity. − S =10s1, 22-fold smaller than measured (Fig. 2A). On the For any given orientation (ψ,θ) of the cell axis, where ψ and θ other hand, B. subtilis differs from a helix in two respects—mo- are polar and azimuthal angles (Fig. S1), this condition yields the tility and the presence of a cell body—suggesting a different drift linear and angular velocities of the bacterium, including the mechanism. Because the body is not chiral, it is not subject to rheotactic velocity vz(ψ,θ). The probability distribution of the cell a lift force and the drag on it acts as an anchor for the drifting orientation, c(ψ,θ), is determined by the competition between flagellar bundle (Fig.
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